The present invention relates to a transfer belt device that transfers a developer image formed on an image bearing member to a recording medium or a transfer belt, and an image forming apparatus provided with this transfer belt device.
In recent years, there has been active development of image forming apparatuses from the two aspects of converting to color and increasing speed. In the colorization of image forming apparatuses, instead of quad-rotational process systems using one image forming station, tandem systems using multiple image forming stations that are able to increase speed have become mainstream.
Tandem image forming apparatuses are provided with a transfer belt device having an endless transfer belt that forms a transport path that transports a recording medium or a developer image, and multiple image forming stations corresponding to each hue arranged in parallel along the transport path, and can perform both color image formation and monochrome (black and white) image formation.
Also, in tandem image forming apparatuses it is possible to set different image forming performance for a color image forming mode that forms color images and for a monochrome image forming mode that forms monochrome images.
For example, there are tandem image forming apparatuses with a configuration wherein the image forming speed is made faster in the monochrome image forming mode, increasing the volume of image formation in monochrome image forming mode. Also, as disclosed in JP 2000-242057A, there are tandem image forming apparatuses with a configuration wherein the diameter of the image bearing member for monochrome images is made larger than the diameter of image bearing members for color images, making the usable time period of the image bearing member for monochrome images equal to that of the image bearing members for color images.
This is because ordinarily, in a single image forming apparatus the frequency of monochrome image formation is higher than that of color image formation. The image bearing member for monochrome images included in the image forming station for monochrome images is used for both color image formation and monochrome image formation, and so its frequency of use is higher than the image bearing members for color images included in the image forming station for color images. Thus, in comparison to the image bearing members for color images, the image bearing member for monochrome images will have a shorter usable time period even if the lifetime is the same.
Accordingly, it is possible to set the specifications for image forming performance, for example changing the speed of image formation in the monochrome image forming mode, for each model of tandem image forming apparatus. Also, in order to set the specifications for image forming performance that differs in each tandem image forming apparatus, specifications are ordinarily set for the color and monochrome image forming stations in each apparatus, and for the transfer belt device, according to the purpose of the image forming apparatus.
On the other hand, the transfer belt device provided in the tandem image forming apparatus is configured from a plurality of rollers that tightly stretch the transfer belt and a transfer unit that supports these multiple rollers. The transfer unit is configured from a plurality of transfer rollers arranged inside the transfer belt. The transfer roller is arranged such that it faces the image bearing member of each image forming station via the transfer belt, and transfers a developer image formed on the surface of the image bearing member of each image forming station onto the outer peripheral surface of the transfer belt or a recording medium transported on the outer peripheral surface of the transfer belt.
In recent years, there have been transfer belt device with a configuration wherein a monochrome image transfer roller and a plurality of color image transfer rollers are raised and lowered for each monochrome image forming mode and color image forming mode, changing the contact states between the transfer belt and the image bearing members in each mode. By doing so, in the monochrome image forming mode, the outer peripheral surface of the transfer belt only touches the monochrome image bearing member due to lowering the monochrome image transfer roller, and only the developer image formed on the surface of the monochrome image bearing member is transferred to the transfer belt.
On the other hand, in the color image forming mode, the outer peripheral surface of the transfer belt contacts the monochrome image bearing member and the color image bearing members due to lowering the monochrome image transfer roller and the plurality of color image transfer rollers, and the developer image formed on the surface of the monochrome image bearing member and the developer images formed on the surface of the plurality of color image bearing members are transferred to the transfer belt one on top of the other.
However, in the transport belt apparatus described above, because the transfer belt is tightly stretched by the plurality of rollers supported by the transfer unit, work to remove the transfer belt from the transfer unit and exchange it with a new one takes time. Accordingly, in the transfer belt device of recent years, as disclosed in JP H8-69238A and JP 2004-109267A, configurations have been adopted wherein a first transfer unit and a second transfer unit are linked together in the axial direction of the plurality of rollers by a parallel axle such that they can swing.
Thus, by swinging the first transfer unit or the second transfer unit into a V-shape, the tightly stretched transfer belt loosens and it becomes easy to exchange the transfer belt.
However, in a transfer belt device provided with a configuration wherein the transfer rollers are raised and lowered in each image forming mode, because the mechanism that raises and lowers the transfer rollers is disposed in a transfer unit, it is difficult to simply adopt the sort of configuration disclosed in JP H8-69238A and JP 2000-109267A. Even when such a configuration has been adopted, it is necessary to swing the transfer unit after first removing the configuration that raises and lowers the transfer rollers, and work to replace the transfer belt becomes troublesome.
It is an object of this invention to provide a transfer belt device wherein even if a configuration is provided that raises and lowers the transfer rollers in each image forming mode, a plurality of transfer units are linked with a simple configuration and can easily swing, making it possible to improve the workability of transfer belt replacement; and to provide an image forming apparatus provided with this transfer belt device.